Do you think it’s important preserve or destroy bad parts of history?

Preserve.

Good or bad is irrelevant as ideas and attitudes shift like fads, albeit more slowly. Haphazardly removing things you find unpleasant is sure to remove things that were good. It's just like clipping pages out of the book. If you clip the villain out, readers will never grasp the full significance of what the protagonist achieved.

Also, history doctoring is a propaganda tool and it's favored by Communists. Communism is a hateful ideology. Tens of millions of its own people have been slaughtered in its name.